1. Field of the Invention
This invention relates to an exhaust gas purification system for an internal combustion engine, particularly to an exhaust gas purification system for an internal combustion engine that has a NOx reduction catalyst (NOx absorber).
2. Description of the Related Art
Recently, the move toward leaner fuel mixture supply controls (in which the air/fuel ratio is regulated leaner than a stoichiometric air/fuel ratio) has created a need for an exhaust gas purification system that has a NOx reduction catalyst (or a NOx absorber) to purify oxides of nitrogen (hereinafter referred to as "NOx") constituent in the exhaust gas, since a greater amount of NOx is generated with the lean fuel mixture.
The NOx reduction catalyst operates to absorb NOx when the engine fuel mixture is set to lean (i.e., in an oxidizing environment where an increased amount of NOx is generated and the oxygen concentration is relatively high) and then to desorb the absorbed NOx when the fuel mixture is rich (where the amounts of hydrocarbon (hereinafter referred to as "HC") constituent and carbon monoxide (hereinafter referred to as "CO") constituent are increased and the oxygen concentration is low). The desorbed NOx is reduced by HC and CO and is emitted into the atmosphere as nitrogen gas, while HC and CO are oxidized and are emitted into the atmosphere as vapor and carbon dioxide.
Since the amount which the NOx reduction catalyst can absorb NOx is limited, as a matter of fact, the lean fuel mixture supply control should not be continued for a long period.
In order to regenerate or renew the NOx reduction catalyst, it has been taught in Japanese Patent No. 2586739, for example, to temporarily enrich the air/fuel ratio beyond the stoichiometric air/fuel ratio to regenerate the NOx reduction catalyst, i.e., to desorb the NOx absorbed into the NOx reduction catalyst so as to reduce it with the rich fuel mixture. This temporary air/fuel ratio enrichment for this purpose is hereinafter referred to as "fuel mixture enrichment for regeneration" or "rich fuel mixture supply control".
Specifically, in this prior art, the amount of NOx absorbed into the NOx reduction catalyst is estimated based on the engine load and engine speed during the lean fuel mixture supply control and the fuel mixture enrichment for regeneration is conducted each time the estimated amount has reached a predetermined amount.
Another Japanese Patent, No. 2692530, discloses a similar fuel mixture enrichment for regeneration. In this prior art, the fuel mixture is temporarily enriched beyond the stoichiometric fuel mixture and is then regulated to the stoichiometric fuel mixture when the engine operation shifts from the lean fuel mixture supply control to the stoichiometric (or therearound) fuel mixture supply control.
However, when the engine operation shifts from the lean fuel mixture supply control to the fuel cutoff control (in which the supply of fuel mixture into the engine cylinder is discontinued), the exhaust gas purification efficiency may sometimes be degraded, unless the fuel mixture enrichment for regeneration is unexpectedly conducted immediately before the fuel cutoff control. This is because the NOx absorbing capability of the catalyst is likely to drop during the fuel cutoff due to the change in the catalyst temperature and some similar factors. Therefore, when the engine operation returns from the fuel cutoff control to the lean fuel mixture supply control, the NOx absorption of the catalyst sometimes becomes insufficient, thereby degrading the exhaust gas purification efficiency.
Moreover, the fuel includes sulfur constituents (hereinafter referred to as "S"). S acts with the oxides and is absorbed as sulfur oxides constituent (hereinafter referred to as "SOx") on the surface or in the micropores of a catalyst, and this also degrades catalyst purification efficiency. In particular, SOx is likely to stick to this type of NOx reduction catalyst and poison the same to degrade the NOx purification efficiency of the catalyst.
The temperature range of the NOx reduction catalyst suitable for NOx absorption or desorption is, approximately from 250.degree. C. to 550.degree. C. The catalyst temperature range suitable for regenerating from sulfur poisoning is beyond the above-mentioned range, and is approximately 600.degree. C. in a rich environment. If the catalyst temperature is raised to approximately 700.degree. C., regeneration from sulfur poisoning will be conducted more effectively. It should be noted that the temperature ranges mentioned above are examples and they depend on the specification of the catalyst.
Such a regeneration from sulfur poisoning is conducted, similarly to the NOx reduction enrichment, by enriching the fuel mixture to raise the catalyst temperature. However, since the aforesaid prior art (i.e., 2692530) is configured such that upon the termination of fuel cutoff, a rich fuel mixture is supplied and a stoichiometric fuel mixture is then supplied, as shown in FIG. 7 of this prior art, it fails to raise the NOx reduction catalyst temperature to a sufficient extent. As a result, this prior art would leave much to improve effective NOx desorption, and the regeneration from sulfur poisoning, if intended, by the same process.